1. Field of the Invention
The invention relates to a high pressure discharge lamp of the direct current operating type.
2. Description of the Related Art
The high pressure discharge lamp of the direct current operating type shown in FIG. 1 is known as a high pressure discharge lamp which is used, for example, for a light source in irradiation with UV radiation. Here, an discharge vessel 91 of fused silica glass has opposite sides of a arc tube portion 91C joined to hermetically sealed portions 91A and 91B. Furthermore, this lamp has a metal foil 92A which is inserted in the hermetically sealed portion 91A of the discharge vessel 91, and a metal foil 92B which is inserted in the hermetically sealed portion 91B of the discharge vessel 91. In addition, this lamp has a pin-shaped anode 93A and a pin-shaped cathode 93B. The anode 93A is attached in the hermetically sealed portion 91A with its base connected to the metal foil 92A, and its tip projecting into the arc tube portion 91C. Similarly, the cathode 93B is attached in the hermetically sealed portion 91B with its base connected to the metal foil 92B, and its tip projecting into the arc tube portion 91C. Still further, this lamp has a supply lead 94A and a supply lead 94B, the supply lead 94A being attached in the hermetically sealed portion 91A with its inner end connected to the metal foil 92A and its outer end projects outwardly from the discharge vessel 91, the supply lead 94B being attached in the hermetically sealed portion 91B with its inner end connected to the metal foil 92B and its outer end projecting outwardly from the discharge vessel 91.
In Japanese patent disclosure document SHO 61-263040 of the assignee of the present application, a technology is disclosed in which, in the above described high pressure discharge lamp of the direct current operating type, there are conductive components on the cathode side on the area of the outer surface of the hermetically sealed portion surrounding the metal foil and these components are used to maintain the hermetic property between the hermetically sealed portion (91B) on the cathode side and the metal foil (92B). In this technology, these conductive components are furthermore electrically connected to the metal foil.
In this technology, the electrical potential of the conductive component during lamp operation becomes the same as the electrical potential of the metal foil. In this way, a hermetic connection between the metal foil and the fused silica glass in the hermetically sealed portion is preserved.
However, in the high pressure discharge lamp known in the prior art, including the lamp described in the above described patent disclosure document, it is considered a disadvantage that the strength of the area of the fused silica glass which forms the discharge vessel and which is in contact with the pin-shaped cathode (the contact surface with the cathode rod and its vicinity) deteriorates over time as lamp operation continues and often fractures originate from this area.
Therefore, it is a good idea to prevent fractures originating from the contact area with the cathode rod and to improve the service life of the high pressure discharge lamp.